Small particles, including microspheres and nanospheres, have many medical uses in diagnostic and therapeutic procedures. Most prior art particles used in medical applications are characterized by numerous disadvantages including irritation of the tissues with which they come in contact and initiation of adverse immune reactions.
Additionally, many of the materials used to prepare the prior art particles may degrade relatively rapidly within the mammalian body, thereby detracting from their utility in certain procedures where long term presence of intact particles may be necessary. Moreover, the degradation of the prior art materials may release toxic or irritating compounds causing adverse reactions in the patients.
It is also a problem in the art for certain types of prior art particles that it is difficult to achieve desirable suspension properties when the particles are incorporated into a delivery suspension for injection into a site in the body to be treated. Many times, the particles settle out or tend to “float” in the solution such that they are not uniformly suspended for even delivery. Furthermore, particles may tend to aggregate or agglomerate within the delivery solution and/or adhere to some part of the delivery device, making it necessary to compensate for these adhesive/attractive forces.
In order to achieve a stable dispersion, it is known to add suitable dispersing agents that may include surfactants directed at breaking down attractive particle interaction. Depending on the nature of the particle interaction, the following materials may be used: cationic, anionic or nonionic surfactants such as Tween™ 20, Tween™ 40, Tween™ 80, polyethylene glycols, sodium dodecyl sulfate, various naturally occurring proteins such as serum albumin, or any other macromolecular surfactants in the delivery formulation. Furthermore thickening agents can be used help prevent particles from settling by sedimentation and to increase solution viscosity, for example, polyvinyl alcohols, polyvinyl pyrrolidones, sugars or dextrins. Density additives may also be used to achieve buoyancy.
It can also be difficult to visualize microparticles in solution to determine their degree of suspension when using clear, transparent polymeric acrylate hydrogel beads in aqueous suspension.
Attempts to use the inert precipitate, barium sulfate, in particle form is known as an additive for bone cement, for silicones for rendering items visible during X-ray examination and for providing radiopacity to polymeric acrylate particles. See Jayakrishnan et al., Bull. Mat. Sci., Vol. 12, No. 1, pp. 17-25 (1989). The barium sulfate also is known for improving fluidization, and is often used as an inorganic filler to impart anti-stick behavior to moist, aggregated particles. Other prior art attempts to increase visualization of microparticles include use of gold, for example, Embosphere Gold™ provides a magenta color to acrylate microparticles using small amounts of gold.
There thus exists in the art a need for small particles that can be formed to have a preferential generally spherical configuration for certain applications such as various therapeutic and diagnostic procedures which are not degraded by the natural systems of the mammalian system, are biocompatible, resist clumping, are easy to visualize in suspension while in use and/or demonstrate acceptable physical and suspension properties.